This invention relates to a hydraulic valve for controlling the flow of fluid under pressure from a source to a using or driven device, and relates more particularly to a direct-drive hydraulic valve.
Hydraulic control valves have long been used to control flows of pressure fluid to various kinds of using devices, one field of particular significance being the aerospace industry where such valves control mechanisms of various kinds in an aircraft. Typically, the valve is interposed between the high pressure source and the using device, and has a movable valve member that is selectively positioned in a stationary valve body to establish fluid flow paths through the valve between the source and the device, the effective areas of the flow paths determining the rates of flow through the valve.
A frequently used type of valve has a sleeve that is mounted in a valve body which defines an elongated bore with a plurality of openings or ports spaced longitudinally of the bore, usually in the form of slots in the sleeve, and a valve spool tightly but slidably fitted in the bore to move back and forth therein and having lands and spool ports for overlying and communicating with different combinations of ports in the valve body. For example, the pressure source may be connected to one or more of the ports in the body, and the using device may be connected to one or more of the other ports, and the spool may have lands and ports for establishing a flow path or paths connecting the source to the device. For a reversible device, alternate paths are established for "forward" and "reverse" operation.
In conventional electrohydraulic valves, a portion of the available pressure fluid usually is used to position the spool in the bore, thereby to control the main portion of the flow to the using device. One difficulty with this type of valve, however, is the loss of some of the available energy when the valve is inactive, through so-called "quiescent flow" in the valve past the spool. Moreover, as the pressure levels used in such systems in the aircraft industry have increased in recent years, quiescent losses also have tended to increase.
Another difficulty that sometimes is encounted in hydraulic control valves is obstruction of movement of the spool by a foreign particle which becomes jammed somewhere between the two relatively movable parts, and which presents a resistance to movement that is greater than the maximum driving force that is available to move the spool, sometimes referred to as the "chip-shearing force". To develop relatively high forces, higher pressures and larger driven surface areas can be used, but these tend to increase not only the energy losses in the system but also the size and weight, which are critical factors in the aircraft industry.
An alternative to a hydraulically driven valve member is a direct drive for the spool, such as a linear electrical motor with a linear variable differential transformer providing feedback for positioning of the spool, or a rotary motor, such as a D.C. torque motor, producing rotary motion at an output shaft that is converted to linear motion of the spool.
Such direct-drive valves have the advantage of consuming driving energy only on demand, have no quiescent flow, and have become feasible as a result of recent increases in the electrical power levels available in the aircraft industry. The principle problem with such valves, however, is the limited amount of chip-shearing force that direct-drive valves are capable of developing with even the higher levels of electrical power that now are available. Claims have been made that chip-shearing forces on the order of eighty pounds can be developed, and forces of this magnitude are regarded as marginal. Moreover, the direct-drive valves that presently are proposed are objectionably large, compared to the electrohydraulic valves that have been in use.
Accordingly, the primary objective of the present invention is to provide a significantly improved driving mechanism which overcomes these deficiencies and is acceptable as a replacement for the electrohydraulic valves that presently are in use.